These studies are designed to evaluate the distribution of myocardial blood flow in the intact unanesthetized dog at rest and during treadmill exercise in the control situation and in the presence of varying degrees of proximal subtotal coronary artery occlusion. Studies will be performed to test the hypothesis that vasomotor activity of the distal coronary vasculature may play an integral role in mediating the especial vulnerability of the subendocardial myocardium to ischemia and infarction. The long-term goal of this research is to produce an experimental model of subtotal coronary artery occlusion associated with enhanced vulnerability of the subendocardial myocardium to ischemia and infarction as seen in the clinical situation.